1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In a conventional image forming apparatus, as a transfer apparatus for electrostatically transferring a toner image on an image carrier onto a printing member, a corona transfer apparatus using corona discharge, a roller transfer apparatus that applies a transfer bias opposite in polarity to a toner onto a conductive elastic roller (i.e. a transfer roller) and electrostatically transfers the toner to a recording material, and a belt transfer apparatus that electrostatically adsorbs a printing member onto a belt-shaped body of rotation and transfers a toner image onto the printing member, etc. are widely used.
In the above mentioned transfer apparatus, the roller transfer apparatus is widely used in recent years, because little ozone can be produced, and the configuration of the image forming apparatus can be simplified in that a transfer roller can also be used as a carrying roller for transporting a printing member.
Transfer bias control (i.e. voltage control) in a roller transfer apparatus will be explained below.
FIG. 11 is a timing chart showing transfer bias control.
In FIG. 11, solid line below shows the magnitude of transfer bias (i.e. voltage) at a transfer position that a toner image is transferred onto a printing member, and a rectangle above shows a position of the printing member. In FIG. 11, a horizontal direction from left to right shows a timing axis.
In FIG. 11, an image forming apparatus is on standby and starts to apply transfer bias to a transfer roller in response to receive such a command showing forming an image from an external apparatus, such as a host computer, and then becomes a state of being capable of forming an image.
However, the start of applying transfer bias to the transfer roller is performed during initial rotation for rotating a photosensitive drum so that surface voltage of the photosensitive drum being on standby remains a constant value.
Then, the image forming apparatus, as an object of applying suitable transfer bias (i.e. voltage) to a transfer roller 5 (see FIG. 1 stated below), controls transfer bias applied to a transfer roller 5 so as to become a predetermined electric current value from an electric current value flowed into a photosensitive drum (i.e. an image carrier) 1 (see FIG. 1 stated below) from a transfer roller 5.
Further, the image forming apparatus determines a transfer bias Vt in the case of transferring a toner image onto the printing member on condition that a transfer bias value for application is set to Vto when an electric current value flowed into a transfer roller 5 becomes a predetermined constant electric current value. However, Vto is 300 VDC to +4.5 KVDC and Vt is approximately +500 VDC to +6.0 KVDC.
In the image forming apparatus, transfer bias (i.e. voltage) applied to the transfer roller 5 is set to Vto until the tip of the printing member P reaches to the transfer position.
Then, transfer bias is changed from Vto to Vt at the timing that the tip of the printing member P reaches the transfer position. This switch timing is switched a little earlier than the timing that the tip of the printing member reaches to the transfer position taking into account a rising characteristic (i.e. a time required from the state of non-applied voltage to applying constant voltage) of a power supply applying transfer bias.
In FIG. 11, the switch timing is switched 30 msec before the timing that the tip of the printing member reaches to the transfer position. However, generally, the switch timing is approximately 10 to 200 ms taking into consideration the rising characteristic of the power supply as well as variation in tolerance on the manufacturing stage of the power supply.
The image forming apparatus changes the transfer bias from Vto to Vt. Thus, the transfer bias is set to Vt when the printing member P is passing through the transfer position (i.e. during transfer period) and is set to Vto after a rear end of the printing member P is passing through the transfer position. The timing that transfer bias is switched from Vt to Vto is equal to the timing that the rear end of the printing member P is passing through the transfer position.
The above-mentioned operation allows the toner image on the photosensitive drum to be transferred onto the printing member P with an optimum transfer bias (i.e. voltage) according to variation in resistance of the transfer roller due to variation in the environment i.e., the operating environment, (such as temperature, humidity) including the image forming apparatus and due to variation in usage of the transfer roller.
In the above operation of the image forming apparatus, the switch timing is set a little earlier (see 30 msec in FIG. 11) than the timing that the tip of the printing member reaches the transfer position taking into account such a variation in the rising characteristic of a power supply applying transfer bias. Thus, larger bias (i.e. voltage) than Vto is applied to the transfer roller 5 after the transfer bias is switched from Vto to Vt until the tip of the printing member P reaches the transfer nip position.
As mentioned above, the image forming apparatus applies the transfer roller 5 to the transfer bias in positive polarity and thus applies directly the transfer bias in positive polarity onto the surface of the photosensitive drum 1 when the printing member P is not positioned at the transfer position.
On the other hand, after the photosensitive drum 1 is passing through the transfer position, the image forming apparatus charges the surface of the photosensitive drum 1 with constant voltage in negative polarity, thereby uniforming voltage on the surface of the photosensitive drum 1 and forming a toner image with a desired density.
However, if the voltage of the transfer bias in positive polarity applied directly to the photosensitive drum 1 is a larger valve, the subsequent charging process occurs a problem (so-called drum memory) that the voltage on the surface of the photosensitive drum 1 cannot be made uniform. Due to drum memory, the voltage on the surface of the photosensitive drum 1 cannot be made uniform in the first charging process, resulting in the difference of density of the toner image the next rotational forming process and causing a notable image defect especially in the case of a half-tone image.
Further, when switch timing that the transfer bias is switched from Vto to Vt is slowed down to prevent the drum memory, a problem occurs on condition that the transfer bias Vt is a large value and the required time for switching the transfer bias is a longer time.
That is, in the case of applying the transfer bias with high voltage, such as the transfer bias Vt applied onto printing member P with high resistance, the transfer bias Vt applied onto the second side of printing member P for double-side printing or the transfer bias Vt applied to low temperature/low humidity environment, etc., a transfer defect occurs due to lower transfer bias at the tip of printing member P.
As described above, in the usage condition, such as the type of printing member P, image pattern (density, printing dot rate, etc.) of a toner image formed on printing member P or environment including the image forming apparatus, etc., it is very difficult to prevent the drum memory as well as a transfer defect at the tip of printing member P.